Immune evasion by HIV-1 involves modulation of class I MHC glycoproteins by Nef. Antigen presentation by infected cells is inhibited due to misrouting of MHC-I from the cell surface to the endo-lysosomal system. This process likely requires formation of a ternary complex involving the cytoplasmic domain (CD) of the MHC-I a chain, Nef, and the endosomal adaptor protein complex AP-1. The roles of Nef and the MHC-I CD in the interaction with AP-1 are not defined, and the structure of the complex is unknown. Preliminary data indicate that fusion of the CD of MHC-I A2 to Nef yields a chimera ("A2-Nef") with AP-binding activity found in neither protein alone. This activity is independent of the canonical AP-binding motif in Nef, as is the modulation of MHC-I in cells; it requires the tyrosine in the MHC-I CD necessary for intracellular modulation by Nef; and it is mediated by the medium ([unreadable]) subunit of AP-1. Based on these data, we propose to use the A2-Nef chimera to determine the molecular basis of the interaction between the MHC-I CD, Nef, and AP-1. First, binding in vitro will be correlated with intracellular function by mutational analysis. Second, the molecular components that are necessary and sufficient for the interaction will be identified using recombinant proteins. Third, the structure of the MHC-I CD/Nef/AP-1 complex will be visualized by x-ray crystallography and validated by mutational analysis. When these studies are completed, we should have a molecular model that reveals how Nef modulates MHC-I by facilitating an interaction between the cytoplasmic domain of the MHC-I a chain and AP-1. One of the ways by which HIV-1 persists in the host to cause AIDS is by evading the immune system via the action of its Nef protein. The current model is that Nef accomplishes this by linking two components of the host cell, named MHC-I and AP-1, to each other. By understanding the molecular basis of this linkage, new approaches to drug design may become apparent. [unreadable] [unreadable] [unreadable]